The Center on Budget and Policy Priorities has published an excellent article about the importance of the Social Security disability program. You can read the entire article, with many charts and graphs, at the site, but here are the opening paragraphs:
Disability Insurance (DI) is an integral part of Social Security. It provides modest but vital benefits to workers who can no longer support themselves on account of a serious and long-lasting medical impairment. The Social Security Administration (SSA) administers the DI program.
In December 2013, 8.9 million people received disabled-worker benefits from Social Security. Payments also went to some of their family members: 160,000 spouses and 1.9 million children.
DI benefits are financed primarily by a portion of the Social Security payroll tax and totaled about $140 billion in 2013. That’s 4 percent of the federal budget and less than 1 percent of the gross domestic product (GDP). Employers and employees each pay a DI tax of 0.9 percent on earnings up to a specified amount, currently $117,000. Financial transactions are handled through a DI trust fund, which receives payroll tax revenues and pays out benefits and which is legally separate from the much larger Social Security retirement fund. Under current projections, the DI trust fund will need replenishment in 2016.
The following charts provide important background information about Social Security Disability Insurance.
Part I: Why Is Social Security Disability Insurance Important?
Part II: Why Have the DI Rolls Grown?
Part III: Who Receives DI?
Part IV: What Financing Issues Does DI Face?